Reversible Capture of Small Molecules On Bimetallaborane Clusters: Synthesis, Structural Characterization, and Photophysical Aspects

Metallaborane compounds containing two adjacent metal atoms, [(PMe2Ph)4MM′B10H10] (where MM′ = Pt2, 1; PtPd, 7; Pd2, 8), have been synthesized, and their propensity to sequester O2, CO, and SO2 and to then release them under pulsed and continuous irradiation are described. Only [(PMe2Ph)4Pt2B10H10], 1, undergoes reversible binding of O2 to form [(PMe2Ph)4(O2)Pt2B10H10] 3, but solutions of 1, 7, and 8 all quantitatively take up CO across their metal–metal vectors to form [(PMe2Ph)4(CO)Pt2B10H10] 4, [(PMe2Ph)4(CO)PtPdB10H10] 10, and [(PMe2Ph)4(CO)Pd2B10H10] 11, respectively. Crystallographically determined interatomic M–M distances and infrared CO stretching frequencies show that the CO molecule is bound progressively more weakly in the sequence {PtPt} > {PtPd} > {PdPd}. Similarly, SO2 forms [(PMe2Ph)4(SO2)Pt2B10H10] 5, [(PMe2Ph)4(SO2)PtPdB10H10] 12, and [(PMe2Ph)4(SO2)Pd2B10H10] 13 with progressively weaker binding of the SO2 molecule. The uptake and release of gas molecules are accompanied by changes in their absorption spectra. Nanosecond transient absorption spectroscopy clearly shows that the O2 and CO molecules are liberated from the bimetallic binding site with high quantum yields of about 0.6. For 3, in addition to dioxygen release in the triplet ground state, singlet oxygen O2(1Δg) was also detected with a quantum yield 2 and CO molecules, from 3 and 4 respectively, is an ultrafast process taking place on a time scale of tens of picoseconds. For SO2, the release is even faster (2 and Pd2 adducts, 5 and 13, the release of SO2 is significantly slower (>1 ns). All these compounds may have potential to serve as light-triggered local and instantaneous sources of the studied gases.